1. Field of the Invention
This invention relates to a vapor phase synthesis method of diamond and diamond obtained by this vapor phase synthesis method and more particularly, it is concerned with a chemical vapor phase synthesis method for producing diamond by using plasma under specified conditions. The resulting diamond can favorably be applied to fields needing higher heat conductivity, smaller dielectric coefficiency, higher transparency, higher specific elasticity, higher strength, higher wear resistance, etc., for example, window materials, oscillation plates, cutting tools, heat sinks, IC bonders, IC substrates, etc.
2. Description of the Prior Art
Up to the present time, diamond has been synthesized in a thermodynamically stable state at a high temperature and high pressure, but of late, it has also been possible to synthesize diamond by a vapor phase synthesis method (CVD method) positively utilizing non-equilibrium state. Actually, a speaker using a dome-shaped oscillation plate of alumina coated with crystalline diamond has been put to practical use.
In this vapor phase synthesis method of diamond, in general, it is proposed to use a hydrocarbon gas diluted with hydrogen in a proportion of 10 times (volume %) as a raw material which is to be excited by a plasma or heat filament. For example, Japanese Patent Laid-Open Publication No. 91100/1983 describes a method comprising previously heating a mixed gas of a hydrocarbon and hydrogen by means of a thermoelectron emission material heated at 1000.degree. C. or higher, feeding the mixed gas over the surface of a substrate and thermally cracking the hydrocarbon to deposit diamond, Japanese Patent Laid-Open Publication No. 110494/1983 describes a method comprising passing hydrogen gas through nonelectrode microwave discharge and then mixing it with a hydrocarbon gas to deposit diamond and Japanese Patent Laid-Open Publication No. 303098/1984 describes a method comprising introducing microwave into a mixed gas of hydrogen gas and inert gas to generate microwave plasma, placing a substrate in the plasma, heating it at 300.degree. to 1300.degree. C. and cracking the hydrocarbon to deposit diamond. The prior art method for the vapor phase synthesis of diamond fundamentally uses, as a raw material, a carbon-containing gas such as hydrocarbon gas and hydrogen gas, as described above, so that the synthesis conditions are limited in such a manner that plasma cannot stably be formed except at a relatively low pressure, for example, up to about 50 torr, and the growth speed is very low.
Furthermore, in Japanese Patent Laid-Open Publication Nos. 158899/1986 and 183198/1986, it is disclosed to add a very small amount of oxygen atom-containing gas to a raw material gas containing carbon and hydrogen so as to improve the crystalline property and growth speed of diamond. However, this method also has disadvantages in that the synthesis conditions, growth speed and synthesis area of diamond are limited because of addition of an inert gas and oxygen atom-containing inorganic gas and deposition of diamond at a low pressure, and accordingly, it is insufficient in applied aspects apart from experimental results.